Columbitrigonia condoni, (PACKARD, 1921)

Halligan, William Keith, 2023, Washington State (USA) trigoniids (Bivalvia) from the conglomerate of Patterson Lake (Early Cretaceous), PaleoBios 40 (8), pp. 1-15 : 6-7

COLUMBITRIGONIA CONDONI ( PACKARD, 1921)

FIG. 6 A-D

Trigonia condoni Packard, 1921, p. 28 , pl.8, fig. 2.

Trigonia packardi Anderson, 1958, p. 109 , pl.1, fig. 5.

Megatrigonia condoni (Packard) . Jones, 1960a, p. 158, pl. 29, figs. 8-10, 17, 18.

Columbitrigonia ? sp. cf. C. condoni (Packard) . Poulton, 1977, p. 14, pl.2, fig. 44.

Columbitrigonia condoni (Packard) . Cooper and Leanza, 2019, p. 27, fig. 3A.

Holotype — Type number 3, Condon Museum, University of Oregon ( Packard 1921). Despite multiple searches, the specimen could not be found . Holotype assumed missing.

Plastoholotype— CASG 567 . Cast of the external mold holotype , RV.

Material— UWBM 112552: Silicone cast from an external mold, LV, from UWBM loc. B-9486. Missing posterior terminus. UWBM 112554: Silicone cast from an external mold, RV, from UWBM loc.B-9487. UWBM 112555: Silicone cast from an external mold, LV, from UWBM loc. B-9487. This specimen demonstrates commarginal lirae not commonly seen on the posterior terminus.

Occurrence— Endemic to the Cordilleran Province ( Cooper and Leanza 2019). Early Albian to Cenomanian(?) ( Jones 1960a). California (?), Oregon, Washington State ( USA), and British Columbia (?) ( Canada) ( Poulton 1977).

Description— Elongate-pyriform shape, small to medium size, moderately inequilateral (umbo anterior), moderately inflated (W/H = 0.22), and longer than high (H/L = 0.67). Umbo moderately prominent with pointed, opisthogyrate beak. Anterodorsal margin almost straight, becoming more convex as it merges with the anteroventral margin. Ventral margin becomes straighter posteriorly. Posterior end narrow and roundly truncated. Posterodorsal margin mildly concave. Area/escutcheon sunken, unornamented except for growth lines, and concave near the umbo, becoming flatter posteriorly; it angles sharply upward to form the posterodorsal commissure. Marginal carina sharp and prominent near the umbo, but posteriorly becomes flatter and less conspicuous.

Ornamentation is variable but generally presents 3 series of ribs. The initial 5-7 umbo riblets are moderately fine, rounded, and concentric with interspaces equal in width to the riblets. The second series consists of 5-7 ribs that begin at the marginal carina, parallel and similar to the initial juvenile series; but soon they flex ventrally, become larger and slightly nodose, forming a gentle anterior curve as they approach the anterior margin at nearly a right angle. The third series consists of 9-11 flank ribs. They all begin subradially, but the first 4-6 anterior ribs in this series soon assume a gentle anterior curve; as these ribs approach the anteroventral margin they enlarge and become more nodose, especially where they intersect with prominent growth lines. Up to the mid-flank point, the flank interspaces are about 2 to 3 times the width of the ribs. The posterior 5-6 subradial flank ribs become straighter, narrower, shorter, and less nodose; they fade as the posterior terminus is approached. The posterior interspaces are about equal in width to the ribs. There is an unornamented space between the marginal carina and the posterior flank ribs. The posterior terminus is generally smooth except for growth lines. However, as seen in UWBM 112555, commarginal riblets may arise from the final two or three posterior-most flank ribs.

Discussion— Columbitrigonia condoni is the second most common trigoniid at this site. The irregular flank ornamentation of these specimens is typical of the species ( Jones 1960a). The only other megatrigoniid from the Cordilleran Province that resembles it is C. jackassensis Poulton, 1977 which is of Hauterivian/Barremian age. They differ in that “the juvenile concentric costae of C. condoni appear to extend to the marginal carina and succeeding subradial costae develop ventrally from them” ( Poulton 1977); the subradial costae of C. jackassensis replace the juvenile concentric costae at an early stage.

Figures

Figure 6.Columbitrigonia condoni (Packard, 1921). All are silicone casts of external molds. A, B. UWBM 112552 from UWBM loc. 9486. Posterior end missing. A. Flank view of LV.B. Oblique view showing areaand marginal carina. C. UWBM 112554 from UWBM loc. 9487. Flank view of RV. D. UWBM 112555 from UWBM loc. 9487. Flank view of LV showing commarginal lirae on the posterior terminus. Scale bars = 10 mm.